Usually, in digital signal processing, a semiconductor memory is used together with a microprocessor. There are two types of semiconductor memories, that is, a volatile memory and a nonvolatile memory. When both types are used together in a single system, the volatile memory is mainly used for temporary storage of data, while the nonvolatile memory is mainly used for storage of programs and fixed values.
In recent years, with improvement in the processability of microprocessors (MRU), a higher rate is required for the volatile memory, and a large capacity is required for the nonvolatile memory. The main type of high rate volatile memory is a synchronous DRAM (SDRAM), and the main type of large capacity nonvolatile memory is a flash memory.
For example, for a DVD (digital versatile disk) driver, on a printed board for signal processing, together with a semiconductor package of an analog front end (AFE) that has the output signal of the optical pickup input to it and performs analog signal arithmetic operation processing, a semiconductor package of a digital front end (DFE) that performs digital signal processing for optical disk reproduction based on a reproduction data signal from the AFE and digital signal processing for recording on an optical disk based on write data from a personal computer or MPEG encoder/decoder (back end), semiconductor packages of a DRAM and flash memory in company with said DFE are also assembled. Here, in the SDRAM, the data immediately after read from the optical disk and the data immediately before write in the optical disk are temporarily stored. Also, in the flash memory, the programs for operation of MPU in the DFE and the various fixed values (such as the setting values for control acquired with the learning function), etc. are unchangingly stored.
In the prior art, with respect to the DFE, the SDRAM and the flash memory are connected entirely individually or independently. Consequently, for example, when the pin numbers of the SDRAM package and the flash memory package are 35 and 32, respectively, it is necessary to allot a total of 67 pins on the DFE package to the SDRAM and the flash memory. As a result, the pin number of the DFE package is increased, and the size of the package becomes larger. Usually, for a DVD drive substrate, the DFE package is significantly larger than the other packages, and it is the determining factor of the substrate size. As a result, as the DFE package becomes larger, the substrate size becomes larger, and this leads to a larger size of the DVD driver. Also, in the future, device integration of the DFE and AFE (to form a single chip or a single package) is a development direction, and reduction of the pin number of the DFE package remains an urgent task to be addressed.
The objective of the present invention is to solve the aforementioned problems of the prior art by providing a type of memory bus shared system characterized by the fact that it can reduce the pin number of a controller that can access both the synchronous memory and the asynchronous memory in parallel or simultaneously, so that it is possible to realize improvement with respect to the package size, substrate size and the assembly cost.
In addition, an objective of the present invention is to provide a type of memory bus shared system characterized by the fact that by using an optimum protocol with respect to the competition between the synchronous memory and asynchronous memory, it is possible to guarantee the access time without a decrease in the data transfer efficiency and data transfer rate for both memories.